The accurate registration of the inkjets in multiple printheads within a printer enables the printer to produce high quality printed images. Registration refers to the positioning and orientation of the printheads with reference to the other printheads in the printer to ensure accurate alignment of the ink drops ejected by the inkjets in one printhead with the ink drops ejected by the inkjets in another printhead. This accurate alignment is required for precise resolution of an ink image as well as reliable and consistent color production arising from the close proximity of ink drops of different colors. During operation of a printer, the positioning and orientation of the printheads are tested from time to time to help ensure proper registration of the printheads. Once the printheads are properly positioned relative to one another, further adjustments can be made to maintain accurate placement of ink drops in the process direction. One of these adjustments includes timing the firing of different inkjets in the printheads to compensate for characteristics in some inkjets that affect the flight of ink drops from the inkjets.
To adjust the timing for the firing of the inkjets, the printer operates the inkjets to form ink marks, also referred to as test patterns, on the surface of an image receiving member. In some printers, the image receiving member is print media, while in other printers, the image receiving member is a rotating endless belt or drum. An optical sensor is positioned opposite the image receiving member. This optical sensor typically includes a plurality of optical detectors that extend across the surface of the image receiving member in the cross-process direction. Each optical detector receives light reflected from a portion of the surface of the image receiving member that is opposite the detector and generates digital image data corresponding to the amount of reflected light received. These digital image data of the image receiving member surface are used to identify the locations of the marks on the image receiving member in both the process direction and the cross-process direction. A digital controller identifies a distance between the actual positions of ink marks and the expected positions of the ink marks and then adjusts the timing of the firing of the inkjets to enable the inkjets to eject the ink drops closer to the expected positions.
While the optical sensor enables the printer to identify registration errors in the printed test patterns, the optical sensor may have defects that produce inaccurate image data. For example, the optical sensor can be skewed at an angle relative to the cross-process direction across the image receiving surface, the optical sensor may deform and bow, and individual optical detectors may be offset in the process direction relative to the other optical detectors in the sensor. An optical sensor that exhibits one or more of these defects generates image data with inaccurate process direction locations of markings in test patterns on the image receiving surface. Thus, the digital controller adjusts the operation of the printheads to correct for the errors identified from the inaccurate data, but these adjustments introduce new errors since the optical sensor generates inaccurate image data for the new ink drop positions. Thus, improvements to inkjet printers to reduce or eliminate errors in image data that are generated due to defects in the optical sensor would be beneficial.